Eur. Phys. J. Appl. Phys. 32, 45–52 (2005)DOI: 10.1051/epjap:2005064 THE EUROPEAN

PHYSICAL JOURNALAPPLIED PHYSICS

Spectroscopic optimization of abnormal glow conditionsfor plasma ion nitriding

A. Qayyum1, R. Ahmad2, A. Waheed3, and M. Zakaullah1,a

1 Department of Physics, Quaid-i-Azam University, 45320 Islamabad, Pakistan2 Department of Physics, G.C. University, Lahore, Pakistan3 PINSTECH, PO Box 2151, 44000 Islamabad, Pakistan

Received: 10 December 2004 / Received in final form: 29 April 2005 / Accepted: 7 July 2005Published online: 15 September 2005 – c© EDP Sciences

Abstract. Optical emission spectroscopy is used to characterize the production of active species as afunction of hydrogen concentration in the mixture under different operating conditions. A major concernis to enhance the concentration of the active species by hydrogen addition that carry several electronvolts energy above their ground states, and thus affect the surface chemistry. The emission intensity ofthe selected optical transitions of molecular and atomic species is measured to determine the functionaldependence of their radiative states. The relative ground state molecular ion density [N +

2 ] is measuredfrom the emission intensity of the first negative band head (λ = 391.4 nm, 0–0) by considering the factthat in low temperature plasma, ion with single charge is produced by the electron impact, and the iondensity is proportional to the electron density. It is found that the concentration of the active speciesmay be enhanced significantly by selecting an appropriate gas composition and operating parameters. TheSS-304 samples are nitrided under the optimum conditions for 4, 8, 12 and 16 hours and hardness valuesare found to increase five times for 16 hours treatment time. The optimized discharge conditions are foundfavorable for plasma ion nitriding.

PACS. 52.80.Vp Discharge in vacuum – 52.70.Kz Optical (ultraviolet, visible, infrared) measurements –81.65.Lp Surface hardening: nitridation, carburization, carbonitridation

1 Introduction

Glow discharge ion nitriding is a surface modification tech-nique, which is primarily used to increase the fatiguestrength, wear and corrosion resistance and surface hard-ness of materials, especially iron-based alloys [1,2]. Usu-ally, abnormal glow regime of the discharge is used for ionnitriding process, because in this operational mode thecathode is fully covered by the glow, which offers the pos-sibility of uniform plasma treatment of the surfaces [3].This technique has recently received considerable indus-trial interest owing to its characteristic of faster nitrogenpenetration without causing any change in bulk propertiesof the substrate, simplicity in application, economic andeasier control of compound and diffusion layers formation.The treatment parameters that can be arbitrarily selectedwithin wide range to produce specific surface structuresand properties make it attractive, compared with otherconventional nitriding methods [4,5]. The addition of hy-drogen with nitrogen enhances the case depth and surfacehardness by removing the surface oxides during the sur-face ion nitriding process. Therefore the compound layerthickness and surface hardness is usually controlled by the

a e-mail: mzakaullah@qau.edu.pk

concentration of hydrogen in the gas mixture [6]. Duringthe ion nitriding process, the reactive species of nitrogenare generated by an electric discharge and are diffusedinto the bulk making the surface hard. The generationsof these reactive species rely on the ability of the plasmato produce a high concentration of excited states of theplasma species. These electronically excited atomic speciesas well as electronically and vibrationally excited molec-ular species carry several electron volts of energy abovetheir ground states and can affect the surface and thusdeposition chemistry [7]. Further, It is widely acceptedthat the nitrogen ions are thermally diffused into the sur-faces resulting in a deep (10–20 µm) nitrided layer [8].These nitrogen ions also deposit energy and momentumand thus contribute to heat the surface to facilitate thefurther diffusion of nitrogen. Moreover, ions in the abnor-mal glow discharge plasma play a major role in generatingthe energetic neutral molecules and radicals bombardingthe substrate that also deposit energy and heat the sur-face [8]. The addition of hydrogen with nitrogen plays animportant role in the ion nitriding process by increasingthe concentration of the active nitriding species in theplasma [9]. Owing to the long distance diffusion of the ni-trogen atoms from surface toward the subsurface regionof the substrate core, two different structures occur in the

Article published by EDP Sciences and available at http://www.edpsciences.org/epjap or http://dx.doi.org/10.1051/epjap:2005064

46 The European Physical Journal Applied Physics

Fig. 1. Schematic illustration of the experimental setup.

nitriding process. The outermost layer, which is a few mi-crometers thick, consists of an intermetallic compound ofiron plus nitrogen (ε-Fe2−3N, Υ -Fe4N) is referred as thecompound layer. Underneath this layer is a significantlythicker diffusion layer, where the nitrogen has mainly beenincorporated into the existing iron lattice as interstitialatoms [10].

The glow discharges used as processing plasma for sur-face treatment can be established and sustained in variousways such as by using pulsed-DC discharges, RF and mi-crowave discharges. However the latter operation modesare not so advantageous for up scaling to large reactors(which is a new tendency in material technology) due tohigh cost of RF and microwave generators. Therefore, thepulsing-DC generated discharges are attracting increasinginterest due to their simplicity and cost effectiveness.

In this paper plasma characterization by means of opti-cal emission spectroscopy is carried out to gain the betterunderstanding of the mechanisms leading to the produc-tion of active species (radicals, atoms and molecular ions),whose abundance determines nitridation process. In par-ticular, the influence of the hydrogen on the occurrence ofthese active species is studied. The main aim of this workis to find optimum operating conditions (gas composition,input power and filling pressure) for the desired treatmentof the samples in a simple and cost-effective manner. Sec-tion 1 reviews the effect of hydrogen admixing into thenitrogen on the production of the active plasma speciesand their role in nitriding process. Section 2 contains thedetails of the experimental procedure along with the di-agnostic details, whereas Section 3 describes the mecha-

nisms of the excitation and the ionization of the plasmaspecies and their radiative processes. The experimental re-sults are presented in Section 4, whereas the concludingremarks are summarized in Section 5.

2 Experimental procedure

The experiment is carried out in a parallel-plate elec-trode configuration consisting of stainless steel electrodeswith a diameter of 7.5-cm and a spacing of 6.0-cm. Theside and back of the electrodes are covered with Ceramiccasing to prevent additional discharge. The electrode as-sembly is housed in a cylindrical stainless steel vacuumchamber of 40-cm diameter and height. The experimen-tal setup is shown schematically in Figure 1. Prior to thefilling of working gases the chamber is evacuated downto 10−5 mbar using a rotary vane pump and oil diffu-sion pump. The flow of nitrogen and hydrogen gases ismonitored with mass flow meters for the desired compo-sition of nitrogen and hydrogen mixture and the pres-sure in the chamber is recorded by using capsule typedial gauge. A pulsing-DC power, which is obtained froma 50 Hz AC power source through the step-up trans-former and the diode chain, is applied to the top elec-trode through the inductive load, which limits the currentduring the discharge. The bottom electrode is grounded,which serves as cathode of the glow discharge. By pow-ering the top electrode with pulsing-DC, N2–H2 mixtureplasma is generated in an abnormal glow regime and op-tical emission spectroscopy of the negative glow region

A. Qayyum et al.: Spectroscopic optimization of abnormal glow conditions for plasma ion nitriding 47

Fig. 2. Emission spectrum recorded from a mixture of 60%H2 + 40%N2 at a filling pressure of 5-mbar and input power of 200watts.

is carried out using a computer controlled system com-prising a McPherson–2061, monochromator having 1200grooves/mm and spectral resolution of 0.01 nm coupledwith a side window photo multiplier tube (PMT-9781B)and auto ranging Pico-ammeter (Keithley-485). The wave-length of the monochromator is calibrated using a mer-cury lamp. The optical emission from the glow dischargeplasma is recorded as a function of hydrogen concentra-tion in the mixture (40–90% H2) for different input pow-ers (200–400 watts) and filling pressures (3–7 mbar). Thefunctional dependence of the population density of ex-cited states and relative density of N +

2 molecular ionsis studied by using the integrated-line intensities of therespective spectral profiles after normalizing for the spec-tral efficiency of the Photomultiplier tube and diffractiongrating. A typical spectrum recorded at a filling pressureof 5-mbar, input power of 200 watts and gas compositionof 60% H2 admixed with 40% N2 is shown in Figure 2. Theemission lines are identified and labeled by using compileddata [11].

3 Spectroscopic diagnostic

Optical emission spectroscopy (OES) is the most populartechnique to investigate the glow discharges since it canbe employed quite simply to obtain information on theconcentration of the species forming the plasma and theirradiative states. The basic premise of this technique isthat the emission intensity of particular wavelength froman excited state is proportional to the concentration ofspecies in that excited state [12]. In the glow dischargethe plasma species are subjected to collisions with elec-trons and with other plasma species. The electron impact

Table 1. Data characterizing the spectral lines of the selectedspecies of atoms, molecules, radicals and ions investigated inthe article.

Spectral line ExcitationSpecies Transitions (nm) threshold (eV)

N2 C 3Πu ⇒ B 3Πg 337.1 (0–0) 11.1

N+2 B 2Σ+

u ⇒ X2Σ+g 391.4 (0–0) 18.7

NH A 3Π ⇒ X 3Σ− 336.0 (0–0) 3.7

H Balmer-β (4,2) 486.13 12.7

Fe z 5F ⇒ a 5D 372.0 3.3

excitation promotes a small fraction of these species intoupper electronic states that decays and emits characteris-tic photons of the plasma species [13], which can be de-tected and analyzed by recording the spectrum. For lowelectron density plasmas (ne < 1011 cm−3), the steadystate coronal model of optical emission is applicable [14].According to this model species are excited solely by elec-tron impact and are lost by radiative decay and not bycollisions with electrons. Therefore the detection of rela-tive intensities of these emissions provides a qualitativeindicator of species concentration. In principle, one canconvert this measurement into a quantitative, relative orabsolute species number density by knowing the EEDFand energy dependent cross sections for the electron im-pact excitation [15]. The spectral lines used in the studyof selected species of atoms, molecules and ions are givenin Table 1 whereas the ionization processes of N2 andH2 species along with their ionization energies is givenin Table 2.

48 The European Physical Journal Applied Physics

Table 2. Ionization processes of N2 and H2 plasma speciesalong with their ionization energies.

Ionization energySpecies Ionization process (eV)

N2 N2 + e− → N +2 + 2e− 15.57

N2 N2 + e− → N+ +N + 2e− 24.5

N N + e− → N+ + 2e− 14.5

H2 H2 + e− → H +2 + 2e− 15.37

H2 H2 + e− → H+ + H + 2e− 18

H H + e− → H+ + 2e− 13.6

3.1 Diagnostic of radiative species

The population of the N2(C 3Πu) excited state is mainlycaused by the direct electron impact excitation (havingenergies above the excitation threshold) from the groundstate of N2(X 1Σ+

g ) [16–18].

N2(X 1Σ+g )ν=0 + e (E > 11.1 eV) ⇒ N2(C 3Πu)ν′=0 + e.

The subsequent radiative decays emit characteristic pho-tons of second positive band head having wavelength of337.1 nm

N2(C 3Πu)ν′=0 ⇒ N2(B 3Πg)ν′′=0 + h ν.

Consequently the emission intensity of the second posi-tive band head is proportional to the population of theN2(C 3Πu) state [17,18]. Explicitly the relation is

I(337.1 nm) ∼ Nu

where I (337.1 nm) is the emission intensity of second pos-itive band head and Nu is the population of the N2(C 3Πu)state. The measurement of the emission intensity of thisband head provides information of the population of theN2(C 3Πu) radiative state in the plasma, which can berelated to the ground state population density as,

N2(C) ∼ [N2(X)]

εmax∫

ε1

ne(ε)σN2(ε)dε

where ne(ε) is the electron energy distribution function,σN2 is the electron excitation collision cross-section at en-ergy ε, ε1 is the threshold energy for the excitation processand [N2] is the concentration of the N2 molecules in thedischarge.

The interpretation of the intensity of the first negativeband head deserves a special discussion. The N2

+(B 2Σ+u )

excited state can be populated either by direct electronimpact excitation

N2(X 1Σ+g )ν=0 + e (E > 18.7 eV)

⇒ N +2 (B 2Σ+

u )ν′=0 + 2e

or stepwise via electron impact ionization of the N2

molecule and then subsequent electron impact excitationof the molecular ion.

N2(X 1Σ+g )ν=0+e (E > 15.57 eV) ⇒ N +

2 (X 2Σ+g )ν′=0+2e

N +2 (X 2Σ+

g )ν=0 + e (E > 3.13 eV)

⇒ N2+(B 2Σ+

u )ν′=0 + e.

The subsequent radiative decays emit characteristic pho-tons of the first negative band head having wavelength of391.4 nm

N2+(B 2Σ+

u )ν′=0 ⇒ N2+(X 2Σ+

g )ν=0 + hν.

Again the emission intensity of the first negative bandhead is proportional to the population of the N +

2 (B 2Σ+u )

state [18]. Explicitly the relation is,

I(391.4 nm) ∼ Nu+

where I (391.4 nm) is the emission intensity of firstnegative band head and N +

u is the population of theN +

2 (B 2Σ+u ) state. For low temperature plasmas (∼3 eV)

the dominant mechanism of the population of N +2 (B 2Σ+

u )state starts from ground state N +

2 (X 2Σ+g ) and the exci-

tation rate coefficients depend very little on the shapeof electron energy distribution function [18]. In contrast,excitation from neutral nitrogen is very sensitive to high-energy tail due to high excitation threshold of about 18 eVand is much suppressed provided the electron tempera-ture is below about 3 eV. This is the basis of ion densitymeasurements [18,20,21]. The population density of theexcited state is related to the ground state population as

N2+(B) ∼ [N2

+(X)]

εmax∫

ε1

ne(ε)σN2+(ε)dε

N +2 (B 2Σ+

u ) ∼ k1ne[N +2 (X 2Σ+

g )].

Considering that in the low temperature plasma,molecular ion with single charge is produced by electronimpact and the ion density is proportional to the electrondensity [22]

i.e. N +2 (X) ∼ ne

N +2 (B 2Σ+

u ) ∼ k1[N +2 (X 2Σ+

g )]2

where k1(10−14 m3 s−1) is the N2+(X) ⇒ N+

2 (B) excita-tion rate coefficient and is very insensitive to the shape ofelectron energy distribution function [18]. Therefore, theemission intensity of the first negative band head shouldbe proportional to the ground state ion density squared[N +

2 (X)]2.

I(391.4 nm) ∼ k1[N +2 (X)]2

I(391.4 nm) ∝ [N +2 (X)]2.

Since the excitation threshold energies for the radia-tive states of NH and Fe are quite low, respectively 3.7and 3.3 eV, therefore electrons directly excite them fromtheir ground states. Consequently, the emission intensityof these radiative states (NH: λ = 336.0 nm and Fe:λ = 372.0 nm) is given by the following relation [16].

I(336.0 nm) ∼ k2ne[NH]

A. Qayyum et al.: Spectroscopic optimization of abnormal glow conditions for plasma ion nitriding 49

I(372.0 nm) ∼ k3ne[Fe]

where [NH] and [Fe] are the ground state densities of NHand Fe respectively, k2 and k3 are the electron impactexcitation coefficients.

4 Results and discussion

The line and band spectra from excited species; N2, N +2 ,

Hβ and NH are observed as a function of discharge pa-rameters. Figure 2 shows that the most intense peak inthe emission spectra comes from the first negative sys-tem, corresponding to the electronic transition from theground vibrational level of the B 2Σ+

u state to the groundvibrational level of the X 2Σ+

g state. Another intense peakcomes from the second positive system, corresponding tothe electronic transition from the ground vibrational levelof the C 3Πu state to the ground vibrational level of theB 3Πg state. Together with these peaks, an intense peakcorresponding to the emission of the NH molecules ispresent. This peak is resulted due to electronic transitionfrom the ground vibrational level of the A 3Π state to theground vibrational level of the X 3Σ− state. The emissionintensity of these peaks gives the population density of therespective excited species.

4.1 Effect of H2 percentage in the mixture

Figure 3 presents that the percentage of H2in the mix-ture affects the emission intensity of molecular and atomicspecies, and consequently the population of their excitedstates. The addition of hydrogen with nitrogen up to70% enhances the population of the N +

2 (B 2Σ+u ) radia-

tive state, which is mainly caused by direct electron im-pact excitation from the ground state of the N +

2 resultingfrom the electron impact ionization of the N2 molecule.Therefore increased emission intensity of first negativeband head N +

2 (B 2Σ+u ⇒X 2Σ+

g (0,0)) with increase inhydrogen concentration from 40–70% illustrates that thetail of the electron energy distribution function (EEDF)is expanded to higher energies and the number of ener-getic electrons is increased. Since the emission intensity ofthe N2 and N +

2 band heads is proportional to the con-centration of these species and the number of electronshaving energy larger than their excitation threshold en-ergies [15]. Therefore the comparison of the N2 and N +

2emission bands also require the consideration of the ex-citation cross-section of the upper levels along with theconcentration of these species. For N2 the threshold ex-citation energy is 11.1 eV, whereas the threshold ioniza-tion energy is 15.57 eV. Therefore ionization cross sec-tion of N2 extends to a larger degree with expansion ofhigh-energy tail of EEDF than the excitation cross sec-tion of N2. Because N +

2 emission is more sensitive tohigh-energy electrons than N2 emission. This illustratesa tendency towards increased population of N +

2 (B 2Σ+u )

state in comparison with N2(C 3Πu) state with addition

Fig. 3. Variation of emission intensity of selected spectral linesof plasma species with H2 percentage in the gas mixture.

of hydrogen up to 70%. Therefore this effect may be ex-plained by an increasing trend of ionization process of N2

molecules into N +2 ions. The dependence of N +

2 (B 2Σ+u )

state population on the gas mixture ratio proposes the op-timal hydrogen percentage ranging from 60–70%. Further-more the population of N +

2 excited state in the plasmacan be controlled by the H2 percentage in the mixture.This effect probably caused by a substantial change inionization mechanisms of the gas mixture, and by possi-ble larger nitrogen-hydrogen ion flux at the cathode, or arise in the secondary electron emission due to interactionsof hydrogen with the cathode surface [23]. The emissionintensity of second positive band head N2(C 3Πu ⇒B 3Πg

(0,0)) decreases with hydrogen concentration in the mix-ture indicating decreased population of N2(C 3Πu) radia-tive state in the mixture.

The emission intensity of the selected Fe line char-acterizes the sputtering of the cathode material, whichis caused mainly by the bombardment of positive ions.The sputtered Fe atoms arrive in the glow discharge andare subjected to collisions with electrons and with otherplasma species. The excitation collisions, and the subse-quent radiative decays, emit characteristic photons of theFe atoms [13]. The intensity of these emitted photons ofcharacteristic wavelength gives the concentration of theFe atoms in the discharge. The emission intensity of hy-drogen Balmer-β line shows the increasing trend with H2

concentration in the mixture.

4.2 Effect of electrical power

Figure 4 depicts the effect of electrical power on the emis-sion intensity of the selected spectral lines for optimizedgas compositions. The emission intensity of N +

2 (B 2Σ+u )

state increases more rapidly than the emission inten-sity of N2(C 3Πu) state up to power of 250 watts andthen decreases. This fact may be explained as follows:the bombardment of positive ions at the cathode does

50 The European Physical Journal Applied Physics

not only releases secondary electrons, but also atomsof the cathode material, which is called “sputtering”.With increase in power, the energy of the secondary elec-trons ejected from the cathode material increases andthe excitation cross-section of N2(C 3Πu) state decreaseswith increase in electron’s energy above the excitationthreshold of N2(C 3Πu) state. The excitation cross-sectionof N +

2 (B 2Σ+u ) state extends to larger degree, because

the excitation of N +2 (B 2Σ+

u ) state is more sensitive tohigh-energy electrons than the excitation of N2(C 3Πu)state [24]. When power is increased above 250 watts thesputtering of the cathode increases, which is characterizedby the increased emission intensity of the selected Fe spec-tral line. The excitation and ionization cross-sections of Featoms are higher for low energy electrons due to lower ex-citation and ionization threshold energies of metal atomscompared with plasma species [25]. Due to ionization andexcitation collisions, the electrons are cooled down affect-ing the emission of N +

2 (B 2Σ+u ) and N2(C 3Πu) states.

The emission intensity of hydrogen Balmer line showsweek dependence on the electrical power.

4.3 Effect of filling pressure

Figure 5 indicates that for the gas composition of 60%H2 + 40% N2, the emission intensity of N +

2 (B 2Σ+u⇒X 2Σ+

g (0,0)) band head increases more rapidly thanthe emission intensity of N2(C 3Πu ⇒B 3Πg (0,0)) bandhead up to filling pressure of 5-mbar, which suggests theincreased population of the N +

2 (B 2Σ+u ) state than the

N2 (C 3Πu) state. Since the population of N +2 (B 2Σ+

u )radiative state is more sensitive to high-energy electronscompared to the population of N2 (C 3Πu) state, thereforethis effect may be explained by the expansion of the high-energy tail of the electron energy distribution function(EEDF) with rise in pressure up to 5-mbar. Above thispressure the emission intensity of N +

2 (B2Σ+u ) state de-

creases whereas the emission intensity of N2(C 3Πu) statecontinues its previous trend. Almost the same trends areobserved for other two gas compositions suggesting the 5-mbar as an optimum pressure for the generation of N +

2molecular ions hitting the substrate. It is observed thatfilling pressure has weak effect on the emission intensityof Fe spectral line in comparison with power.

4.4 Measurement of relative ion density [N +2 ]

Molecular ion density is an important character of plasma,which deposits energy and momentum and thus con-tributes to the self-discharge heating of the surface forthermal diffusion of nitrogen. The band head intensity ofthe transition 0–0 (λ = 391.4 nm) belonging to first nega-tive system is used to determine the functional dependenceof the relative molecular ion density [N +

2 ] on the hydrogenfraction in the mixture for different operating parameters.The relative dependence of molecular ion density [N +

2 ] ismonitored by taking into account the fact that in the low

Fig. 4. Variation of emission intensity of selected spectral lineswith input power for various H2–N2 gas mixture compositions.

temperature plasmas (below 3 eV), the dominant mecha-nism of the population of N +

2 (B 2Σ+u ) state starts from

ground state N +2 (X 2Σ+

g ) mainly produced by electronimpact and the excitation rate coefficients depend verylittle on the shape of electron energy distribution function(EEDF). Moreover, ion with single charge is produced byelectron impact and the ion density is proportional to theelectron density [18]. Figure 6 depicts the increase in N +

2

A. Qayyum et al.: Spectroscopic optimization of abnormal glow conditions for plasma ion nitriding 51

Fig. 5. Variation of emission intensity of selected spectral lineswith filling pressure for various H2–N2 gas mixture composi-tions.

concentration with the addition of H2 up to 70%, whichis generally explained as a cathode surface effect provid-ing arise in the secondary electron emission that generateions by inelastic collisions [16]. This effect may be dueto surface cleaning by hydrogen, which removes the sur-face oxides. The molecular ion density increases with in-put power up to 250 watts, which may be explained bythe increase in energy of the secondary electrons ejectedfrom the cathode material due to the cathodic bombard-

Fig. 6. Relative dependence of molecular ion density [N +2 ]

on: (a) H2 percentage in the gas mixture, (b) input power, (c)filling pressure.

ment [24]. These energetic electrons make inelastic colli-sions with the nitrogen molecules and ionize them. Thedecrease in the ion density at elevated power may be dueto the diffusion of the energetic electrons to the wall ofthe chamber, which reduce the energy of the bulk elec-trons. The increase in ion density with filling pressure upto 5-mbar may be due the pressure dependent change inthe ion mobility caused by hydrogen addition, which af-fect the excitation and ionization mechanisms [23]. The

52 The European Physical Journal Applied Physics

Fig. 7. Variation of relative increase in surface hardness withthe treatment time (hardness of treated samples/hardness ofuntreated sample).

molecular ion density as obvious from the results has sig-nificant dependence on the gas composition and operatingparameters, and ion flux to the substrate can be optimizedby their appropriate selection.

4.5 Surface hardness of SS-304 samples treatedunder optimum conditions

Spectroscopic measurements reveal that 70% hydrogen ad-mixed with 30% nitrogen at 5-mbar filling pressure and250 watts input power may be optimum for the produc-tion of active species for surface treatments. The SS-304samples are nitrided under these optimum abnormal glowconditions for different duration of times. The surfacehardness of the treated samples is measured by using Vick-ers hardness testing system. It is found that the surfacehardness of SS 304 increases linearly with treatment timeand is enhanced up to five times for a treatment time of16 hours. The results are presented in Figure 7.

5 Conclusions

The emission intensity of the selected spectral lines is re-lated to the population density of their radiative statesby considering the fact that in low temperature plasmasthe dominant mechanisms of excitation and ionization aremainly caused by electron collisions. A major concern is toenhance the concentration of the active species by hydro-gen addition for plasma surface treatments. The emissionintensity of the selected optical transitions of molecularand atomic species is measured to determine the func-tional dependence of their radiative states. The relativemolecular ion density [N +

2 ] is measured from the emissionintensity of the first negative band head (λ = 391.4 nm,0–0) considering the fact that in low temperature plasma,ions with single charge is produced by the electron impact,and the ion density is proportional to the electron density.

It is found that the generation of the active species maybe optimized for the desired treatment of the samples byselecting an appropriate gas composition and operatingconditions. The optimized discharge conditions are foundfavorable for ion-nitriding.

The work was partially supported by the Ministry of Scienceand Technology Grant, Pakistan Science Foundation ProjectNo. PSF/R&D/C-QU/Phys. (199), ICSC- World LaboratoryProject E-13 CHEPCI Islamabad, Quaid-i-Azam UniversityResearch Grant and Higher Education Commission ResearchProject for Plasma Physics. We are thankful to the referee forhis valuable suggestions to improve the manuscript.

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